The fabrication of many photonic devices has been achieved through exposure of transmissive and absorbing materials to intense laser radiation in order to change the optical properties of said materials. For example, UV-induced photosensitivity of germanium doped silica glasses has been exploited in order to create permanent refractive index changes in the photosensitive Ge-doped silica cores of single mode optical fibers and waveguides as opposed to the undoped cladding. By creating a spatial intensity modulation of the UV exposure either by using a two-beam interference technique as disclosed in U.S. Pat. No. 4,807,950 by Glenn et al. or by using a phase mask as disclosed in U.S. Pat. No. 5,367,588 by Hill et al., Bragg grating structures can be produced in the photosensitive core of the waveguide.
Bragg gratings in optical fiber and waveguides have developed into an important technology for wavelength division multiplexing (WDM) systems and other applications for fiber optic systems such as optical sensing because of the highly desirable optical characteristics the Bragg structures exhibit as well as the relative ease with which they can be fabricated. A large variety of optical devices have been fabricated using Bragg gratings in waveguides including optical add/drop multiplexing filters (OADM), gain flattening filters, band splitters and dispersion compensators.
As disclosed by Glenn et al., permanent periodic gratings are provided or impressed into the core of an optical fiber by exposing the core through the cladding to the interference fringe pattern generated by two coherent beams of ultraviolet laser light that are directed against the optical fiber symmetrically to a plane normal to the fiber axis. The material in the fiber core is exposed to the resultant interference fringe intensity pattern created by the two overlapping UV beams creating permanent periodic variations in the refractive index along the length of the UV photosensitive core of the waveguide. The resultant refractive index variations are oriented normal to the waveguide axis so as to form the Bragg grating.
A more popular method of photo imprinting Bragg gratings is taught by Hill et al. in U.S. Pat. No. 5,367,588 where an interference fringe pattern is generated by impinging a single UV light beam onto a transmissive diffractive optic known as a phase mask. The waveguide to be processed is placed immediately behind the phase mask and is exposed to the generated interference fringe pattern leading to the formation of the Bragg grating structure. In these prior art examples, optical fibers or waveguides having a Ge doped photosensitive core are irradiated with UV light at a predetermined intensity and for a predetermined duration of time sufficient to obtain a substantially permanent Bragg grating structure within the core of said waveguide.
These prior art gratings provide a useful function, however they are known to suffer from some limitations in terms of the out-of-band loss that results from coupling of the fundamental core mode LP01 into backward-propagating lossy cladding modes. For example, a single mode optical fiber with a UV-photosensitive core and non UV-photosensitive cladding, will develop a Bragg grating structure using the techniques disclosed by Glenn et al. and Hill et al. only in the core region of the fiber. The fiber Bragg grating will reflect light in a narrow band centered on the Bragg resonance wavelength, λBragg, determined by the well-known Bragg's diffraction conditionmλBragg=2neffΛ  (1)where neff is the effective refractive index seen by the fundamental LP01 core mode, Λ is the period of the grating and m is the order number. Re-expressing equation (1) in terms of the propagation constant of the fundamental mode β01=2πneff/λBragg yields the phase matching condition
                              2          ⁢                      β            01                          =                              2            ⁢            π            ⁢                                                  ⁢            m                    Λ                                    (        2        )            
Because the refractive index modulated grating is localized to the core region only, but the mode field of the LP01 extends into the unmodulated cladding region, some coupling of the energy into cladding modes occurs due to a non-zero weighted overlap integral between the guided LP01 mode and the cladding modes. For optical fiber waveguides that comprise a core, a cladding surrounding the core and then air or a protective coating surrounding the cladding, several propagation modes can be supported in the cladding region. These modes may be guided or lossy depending on whether the outer layer surrounding the cladding has a lower or a higher refractive index than the cladding. These modes are commonly referred to as LPμv cladding modes where μv is the mode number. If the phase matching conditionβ01(λμv)+βμv(λμv)=2π/Λ  (3)
is satisfied, light propagating in the LP01 mode may couple into cladding mode LPμv, where βμv=2πnμv/λμv is the propagation constant of cladding mode LPμv at wavelength λμv, nμv is the modal refractive index of the cladding and β01=2πneff/λμv is the propagation constant of the fundamental mode LP01 at wavelength λμv. Since nμv is always less than neff for a single mode optical waveguide, the wavelength λμv at which the phase matching condition in equation (3) is satisfied will always be less than the Bragg resonance wavelength λBragg.
Usually there are a series of wavelengths that meet this condition, corresponding to a series of cladding modes. Power that is coupled into the cladding modes is not entirely guided hence it is lost through absorption or scattering through the fiber coating.
The strength of coupling between the LP01 mode and the various cladding modes LPμv, caused by the grating can be measured by a coupling coefficient, containing an overlap integral performed over the cross section of the fiber as discussed by Erdogan in “Fiber Grating Spectra” J. Lightwave. Tech. 15 (8), p. 1277–1294 (1997)
                              κ                      01            ,                          µ              ⁢                                                          ⁢              v                                      =                              ω            4                    ⁢                                    ∫              A                        ⁢                                          ∫                ∞                            ⁢                                                          ⁢                                                ⅆ                  x                                ⁢                                                                  ⁢                                  ⅆ                  y                                ⁢                                                                  ⁢                                                      E                    01                                    ⁡                                      (                                          x                      ,                      y                                        )                                                  ⁢                                                      E                                          µ                      ⁢                                                                                          ⁢                      v                                                        ⁡                                      (                                          x                      ,                      y                                        )                                                  ⁢                Δ                ⁢                                                                  ⁢                                                      n                    f                                    ⁡                                      (                                          x                      ,                      y                                        )                                                                                                          (        4        )            
where κ01,μv is the coupling coefficient between the guided LP01 mode and a cladding mode LPμv. A∞, ω, E01(x,y), Eμv(x,y) and Δnf are the fiber cross section, angular optical frequency, guided mode field, cladding mode field and photosensitivity profile respectively. The integral in equation (3) is nulled, ie κ01,μv=0, if Δnf is constant over the area where the guided LP01 mode field is confined. This is usually not the case in standard photosensitive core fibers since the cladding is far less photosensitive.
In a method disclosed by E. Delevaque et al, Conference on Fiber Communication, Technical Digest Series, Vol 8, No. 6, pp 343–346 (1995), the cladding is rendered photosensitive as well as the core, so that the refractive index grating is recorded in both the core and, to an extent, in the cladding. When a UV-induced index grating is written into the core and the intermediate cladding region, suppression of cladding modes results.
There are several examples of prior art where different fiber designs and fiber chemistries are employed to render the cladding photosensitive to UV light, as disclosed in U.S. Pat. Nos. 5,627,933 and 5,790,726 by Ito et al., U.S. Pat. No. 6,005,999 by Singh et al., U.S. Pat. No. 6,009,222 by Dong et al., U.S. Pat. No. 6,221,555 by Murakami et al. and U.S. Pat. No. 6,351,588 by Bhatia et al. These prior art fiber designs provide a useful function however they suffer from some limitations. Often the complicated fiber chemistry results in a fiber that possesses many internal stresses, which makes the fiber more fragile. Mode fields of the guided LP01 are not identically matched to standard Ge-doped telecom fiber (for example SMF-28 from Corning Inc.) hence high splice losses result. The fiber is often more expensive to produce.
These prior art gratings produced in these UV-photosensitive cladding fibers often suffer from some limitations in terms of the amount of induced index change that is possible. In order for high refractive index modulated Bragg grating structures to be written in these optical fibers, the optical fiber often needs to be photosensitized to UV light by exposing such an optical fiber to hydrogen or deuterium gas at elevated pressures and temperatures as taught by Atkins et al. in U.S. Pat. No. 5,287,427 or by hydrogen flame brushing as taught be Bilodeau et al. in U.S. Pat. No. 5,495,548. After exposure, it is preferable to subject the UV written structures to annealing at elevated temperatures in order to remove any remaining interstitial hydrogen or deuterium present in the waveguide core. As taught by Erdogan et al. in U.S. Pat. No. 5,620,496, this annealing step is often implemented in order to stabilize by accelerated aging, the induced index change. These extra processing steps to the optical fiber or waveguide complicate the manufacturing of photonic devices and reduce yield.
Recently processes that employ high-intensity laser pulses in the femtosecond pulse duration regime for creating permanent changes in the refractive indices of glasses have been explored by several groups of researchers. K. M. Davis et al. disclose a technique for inducing index change in bulk glasses with ultra-high peak power femtosecond infra-red radiation in Opt. Lett. 21, 1729 (1996), incorporated herein by reference. The physical process that appears to cause the refractive index change in the materials need not be due to the dopant dependant mechanisms occurring with UV-induced index change, namely color center formation. Instead the refractive index change is due to the creation of free electrons through non-linear absorption and multi-photon ionization of bound charges, followed by avalanche ionization and localized dielectric breakdown as these free electrons are accelerated by the intense but short time duration laser field. Also, this leads to a localized melting and restructuring of the material and a concurrent increase in the index of refraction. The creation of waveguides in bulk glasses using this technique is taught by Miura et al. in U.S. Pat. No. 5,978,538 while the modification or trimming of existing waveguide structures is taught by Dugan et al. in U.S. Pat. No. 6,628,877; both of these references are incorporated herein by reference.
In order to photo imprint retroreflective Bragg structures into the core of optical fibers or waveguides using high-intensity femtosecond time duration radiation, it is advantageous to generate an interference fringe pattern originating from a single femtosecond laser pulse either using a holographic technique or a diffractive optic. Hosono et al. in U.S. Pat. No. 6,633,419 incorporated herein by reference disclose an apparatus for producing a hologram using a two-beam laser interference exposure process, comprising the steps of using a femtosecond laser having a pulse width of 10 to 900 femtoseconds and a peak output of 1 GW or more that is capable of generating a pulse beam at or close to the Fourier transform limit. The beam from the laser is divided into two beams using a beam splitter, controlled temporally through an optical delay circuit and spatially using plane and concave mirrors each having a slightly rotatable reflection surface to converge the beams on a surface of or within a substrate for recording a hologram at an energy density of 100 GW/cm2 or more with keeping each polarization plane of the two beams in parallel so as to match the converged spot of the two beams temporally and spatially, whereby a hologram is recorded irreversibly on the substrate formed of a transparent material, semiconductor material or metallic material. The volume hologram is optionally layered so as to provide a multiplex hologram recording that is permanent unless it is heated to a temperature to cause the structural change in the atomic arrangement of the substrate in which the hologram is inscribed.
Miller et al., in U.S. Pat. No. 6,297,894 incorporated herein by reference, teach a method for utilizing a diffractive optic to generate an interference fringe pattern in order to induce refractive index changes in materials using femtosecond time duration laser radiation. An exemplary embodiment of the invention of Miller et al. comprises a femtosecond laser source for providing light to a diffractive optical element. Light propagating from the diffractive optical element is incident on a curved mirror, which acts to focus the light into a lens or another curved mirror and then into a target.
Mihailov et al. in U.S. patent application Ser. No. 10/639,486, from which this application claims priority, disclose a technique for fabrication of Bragg grating structures in optical media such as optical fibers and waveguides with an ultrafast (<500 ps ) laser source and a zero-order nulled phase mask using a direct writing technique. The resultant grating structures have high induced-index modulations (>1×10−3) which were achieved without any special fiber sensitization process such as those taught be Atkins et al. in U.S. Pat. No. 5,287,427. Since the refractive index change need not be dependent on the dopant in the core or cladding of the waveguide, refractive index changes can be induced in both regions of the waveguide.
It is an object of this invention to overcome the aforementioned limitations within the prior art systems for fabrication of cladding mode suppressed Bragg gratings in optical fiber and waveguides by inducing refractive index change in optical fibers and waveguides using femtosecond time duration laser radiation. Additionally, it would be beneficial to provide a simple method of producing high quality fiber Bragg gratings (FBGs) that are robust, do not require specialty optical fiber and are not subject to photosensitization techniques or annealing.
It is a further object of this invention to provide a method of writing a grating in the cladding of a standard optical fiber that has not been photosensitized and that is not necessarily sensitive to actinic radiation.